A phase relationship may exist between cardiac cycle frequency and breathing cycle frequency in a person, with the breathing cycle frequency being somewhat less than the cardiac cycle frequency. The chemical makeup of the blood, e.g., the oxygen and carbon dioxide concentrations within the blood, may also have some phase relationship with respect to blood flow and the phase of the breathing cycle. Respiratory sinus arrhythmia (RSA) is a phenomenon that reflects such a phase relationship between the breathing cycle and the cardiac cycle. RSA describes a variation in heart rate that occurs during a breathing cycle. During RSA, a person's heart rate may accelerate during inhalation and decelerate during exhalation. RSA may improve pulmonary gas exchange, and may be more efficient in cardiopulmonary energy consumption compared to a constant heart rate. In some examples, RSA may be more prevalent in a younger person, and may be lost as the person ages, or lost due to other pathology.